(1) Field of the Invention
This invention relates to an improvement in the process for making polymethylene polyphenyl polycarbamate, from N-phenyl carbamate and formaldehyde as the starting materials.
More particularly, the invention relates to an improved process for making polymethylene polyphenyl polycarbamate through the reaction of N-phenylcarbamate with formaldehyde in the presence of an acid catalyst, the characteristic feature residing in that the quality and yield of the object product are markedly improved by practicing the reaction in the concurrent presence of a compound or compounds having specific structures. The invention also relates to an improved production process, which is characterized in that, when the reaction is effected in the presence of an aqueous acid solution as the catalyst, the aqueous acid solution separated and recovered from the reaction system after the reaction is repetitively re-used many times under specific conditions.
(2) Description of the Prior Art
Polymethylene polyphenyl polycarbamate is a valuable substance as a starting material for agricultural chemicals and medicines, polyamide and polyurethane. Upon pyrolysis, polymethylene polyphenyl polycarbamate can also produce the corresponding polymethylene polyphenyl polyisocyanate. The development of an industrially advantageous method for its production has therefore been much awaited.
It is known to react polymethylene polyphenyl polyisocyanate with an alcohol to produce the corresponding polymethylene polyphenyl polycarbamate. This method however requires the use of strongly toxic aniline or phosgene for making the starting polymethylene polyphenyl polyisocyanate, and the method itself is very complex.
Also a process for reacting polymethylene polyphenyl polyamine with an alkyl ester or chloroformic acid to make the corresponding polymethylene polyphenyl polycarbamate has been known. Both of the starting materials, i.e., polymethylene polyphenyl polyamine and alkyl chloroformate exhibit strong toxicity or irritating property and are difficult of handling. The process also is complicated, and cannot be said an industrial process.
As a still another process for making polymethylene polyphenyl polycarbamate, that reacting N-phenyl carbamate with formaldehyde is known. For example, West German Pat. No. 1,042,891 and U.S. Pat. No. 2,946,768 disclose the formation of a condensation product by heating N-phenyl carbamate and formaldehyde together with an aqueous hydrochloric acid. The literatures however are entirely silent on the structure and composition of the product. In our reproduction of the last process, the reaction progressed very slowly, and the product contained large amounts of unreacted N-phenyl carbamate and by-products. Naturally the yield and the selectivity for the object product were low, and the process was found to be inadequate as an industrial process for making polymethylene polyphenyl polycarbamate.
It has also been recently disclosed by Japanese Laid-Open patent application No. 59264/1979 (U.S. Pat. No. 4,146,727) that upon reacting N-phenyl carbamate with formaldehyde by the above-described process, aminobenzylanilines are by-produced besides polymethylene polyphenyl polycarbamate, and when the reaction product containing the by-product is heated to 50.degree.-170.degree. C. using a protonic acid or a Lewis acid in anhydrous state, the aminobenzylanilines can be converted to polymethylene polyphenyl polycarbamate. By that method, however, first large quantities of by-products are formed because the condensation product is made by the heretofore known process, and, second, the condensation product must be dewatered and subjected to the rearrangement reaction under rigorous conditions. The two stage process requires complex procedures and is defective as an industrial process. Furthermore, although it is true that the aminobenzylanilines are decreased in the later stage rearrangement reaction, they are not necessarily effectively converted to the object product, but the greatest part is pyrolyzed to form high molecular polymers inadequate for obtaining polyisocyanate of high quality. Thus the intended product of satisfactory quality cannot be obtained.